The present invention relates to the field of measuring samples using Terahertz radiation. More specifically, the present invention relates to measuring samples which have an orientation which is not controlled for a Terahertz measurement.
THz radiation has been employed to measure and characterise many different materials such as pharmaceutical products, coating layers, paint, chemicals etc. Terahertz technology allows for the first non-destructive imaging of chemical and/or structural features of the pharmaceutical tablet containing the active pharmaceutical ingredient (API) and excipients (fillers).
Typically a THz beam is launched at the sample and either the reflected or transmitted beam is analysed to yield pertinent metrics about the sample. Many important materials are transparent to THz radiation and have important characteristic spectral features in the THz frequency range. Therefore the internal structure and chemical composition of the sample may be measured.
These measurements are usually conducted with the sample held stationary in a well defined orientation. This is to optimise the collected radiation for maximum quality of signal. This is because the THz specular reflectance of many interesting materials is relatively low <10% and the diffuse reflectance is ˜three orders of magnitude below the specular reflectance. The situation is further complicated by the relatively weak signals that are produced and detected by THz devices. Often a well orientated stationary sample is needed to produce any signal at all. Furthermore, weak signals need to be averaged over a time that is longer than the tablet motion.
An example of an application of THz technology is to generate non-destructive images of internal structures within pharmaceutical solid dosage products. One of these internal structures is the outer coating of the product, designed for a number of different functions; cosmetic, control release of the active ingredient and protection of the active ingredient from the atmosphere. The uniformity of the coating can critically affect the bio availability (rate of absorption of a drug by the body and therapeutic efficacy of the accompanying dosage form) to patients. Process control during the coating stage is thus critical, but is currently limited to remote analysis.
One proven approach utilises the current Terahertz Pulsed Imaging (TPI) technology, which uses a ‘time-delay’ sampling technique to measure the information contained within the pulse shape itself, and not just the reflected power. Another approach utilises the current Terahertz Pulsed Imaging (TPI) technology ‘time-delay’ sampling technique to measure the peak amplitude of the reflected THz pulse.
In all of these examples a well defined special relationship between the THz pulse (i.e. that delivered by the THz optics) and the sample position is required. The rate at which a spectra or image may be obtained is thus limited by the mechanical arrangement of the delivery system—for example a motorised stage or a robotic handling arm.
However, many industrial processes produce large numbers of continuously moving, rotating and randomly orientated samples. These measurement scenarios present a challenge for sample measurement using THz radiation as the position, orientation and velocity of the sample cannot be guaranteed during the measurement time. The data collected is usually confused with the added parameters of the sample motion, position and orientation.